1. Field of the Invention
The invention is directed to a local oscillator for generating a narrow-band high-frequency signal (HF signal) for direct signal mixing with a reception signal generated by a reverse-biased avalanche photodiode (APD) from a light signal impinging on the APD.
2. Description of the Prior Art
Avalanche photodiodes (APDs) are used in sensitive optoelectronic measurement systems, among others. For example, optoelectronic measurement of distances up to 100 m with accuracies of a few millimeters has great importance for numerous applications, particularly in the construction industry and in plants engineering. Distance measuring systems of this kind should be as highly dynamic as possible so that very weak, as well as very strong, light reception signals can be processed. This renders superfluous the use of defined target marks on the object whose distance from a reference point is to be determined. The possibility of direct distance measurement at determined surfaces, i.e., without the use of target marks, makes it possible, particularly in the technical fields and industries mentioned above, to reduce manufacturing time and to cut costs while simultaneously reducing manufacturing tolerances.
In order to ensure the required high dynamics and in order to detect very weak measurement signals, the use of sensitive APDs is often unavoidable and is also known in principle in methods and arrangements for high-precision optoelectronic distance measurement. In distance measuring arrangements, in most cases, a preferably sinusoidal intensity-modulated beam of a light source, particularly a laser diode, is directed to a measurement object (EP 0 701 702 B1, DE 196 43 287 A1, U.S. Pat. No. 4,403,857). The intensity-modulated light which is reflected or backscattered from the measurement object is detected by a photodiode. The distance to be measured is given by the phase displacement—relative to the emitted light intensity of the light source—of the light intensity which is modulated in sinusoidal manner and backsctattered from the measurement object. For successful elimination of phase errors depending on temperature, aging and reception power in high-precision distance-measuring and phase-measuring systems, it was suggested in German Patent Application 100 06 493.0, as an improvement of the measurement arrangement according to U.S. Pat. No. 4,403,857, to modulate the light intensities of a main emitter and reference emitter simultaneously with different modulation frequencies and to generate a signal mixture which is based on the use of APDs as main receiver and reference receiver and which contains a signal with the intensity modulation frequency of the main emitter and a signal with the intensity modulation frequency of the reference emitter and a signal with the intensity modulation frequency of the reference emitter. A trouble-free distance measurement with definite distance information is made possible by means of simultaneous measurement of the phases of the two signals generated in this way and the separation of the two phases in an intermediate frequency range.
Since very weak signals must be taken into account with large distances and in technical measurement object surfaces, i.e., without the use of target marks, it is necessary to use sensitive APDs which, for example, in comparison to the PIN photodiodes, cause an additional internal amplification of the photocurrent generated by the incident light power. This inner amplification results from a charge carrier multiplication in the avalanche zone of the APD in which a high electric field strength exists. The charge carriers generated by the incident light are highly accelerated by this field strength, so that, as a result of their high energy state, they release additional charge carriers from the semiconductor material of the APD resulting in an additional amplification of the photocurrent.
A high voltage is required in the blocking direction in order to generate the high electric field strengths in the avalanche zone of the APD. Depending on the type of APD, this voltage ranges from 40 V to 500 V. Typical gain factors of the photocurrents are between 10 and 200. They are highly dependent on the semiconductor material (e.g., Si, InGaAs), the construction of the photodiode, the blocking voltage and the temperature.
A problem, already mentioned, with the type of optoelectronic distance measurement described above, especially on technical or engineering surfaces, is the detection of very weak signals. In this connection, only very slight interference noise and very weak electric crosstalk (e.g., <110 dB) from the light transmitter (laser) to the photodiode receiver may exist in the measuring system.
In order to ensure very weak crosstalk and the least possible in-coupling of external interference fields (radio fields, digital interference), a method of direct signal mixing may be applied in the receiver as is known, for example, from U.S. Pat. No. 4,503,857, cited above, and from a scientific paper by K. Seta, T. Oh'Ishi, “Distance Measurement Using a Pulse Train Emitted from a Laser Diode”, Japanese J. of Appl. Physics, Volume 26, No. 10, pages L1690-L1692, October 1987, and as also has been suggested in an especially advantageous modification in German Patent Application 100 06 493.0, also cited above. With this kind of direct mixing, the (preferably) sinusoidal signal of frequency fLO of a local oscillator and an amplitude of >1 V are superimposed on the APD blocking voltage, so that along with the blocking voltage the gain factor M of the APD, i.e., its internal current source, is also modulated. The following equation in a first approximation applies to the APD output current: iAPD(t)=M(t)·iFOTO(t), where M(t) is the modulated APD gain depending on time t and iFOTO(t) is the internal photocurrent generated by the light incidence. Because of the nonlinear relationship between the APD gain and the internal photocurrent, a mix product results, i.e., an intermediate frequency signal (IF signal), which oscillates with the frequency difference between the frequency fLO of the local oscillator and the frequency fMESS of the modulated detected light output. The frequency conversion accordingly takes place in the internal current source of the APD. By means of low-pass filtering, higher-frequency components are eliminated. The output signal of the APD, i.e., the IF signal, is (comparatively) low-frequency and can accordingly be further processed without difficulty. Since the mixing process takes place within the chip of the APD, the structural sizes of the mixing arrangement are typically smaller than the utilized modulation wavelength by three or four orders of magnitude. Consequently, external electromagnetic interference and electrical crosstalk are negligible to a great extent. Interference output also leads, in principle, to increased noise. By means of the described step, the noise characteristics are also clearly improved. It is also advantageous that the IF signal arising from the direct mixing is, as a rule, comparatively very low-frequency; e.g., 10 kHz to 100 kHz. No interference is to be expected in this frequency range. Also, parasitic characteristics of other electronic components are negligible at these low frequencies. Since the output signal of the APD is in the IF range, no other high-frequency components are needed in the reception part aside from the local oscillator. Accordingly, production costs as well as power consumption of an optoelectronic distance measuring system outfitted with APDs of this type can also be drastically reduced.
However, this results in a problem which is the object upon which the invention is based: due to the desired—and, in principle, also achievable—low power consumption of the distance measurement device which, in general, is to be operated from a battery which is as small as possible, it is not possible to use known commercially available PLL (Phase Locked Loop) oscillators followed by a 50-ohm HF amplifier for generating the local oscillator signal for frequency mixing or, if so, only with unsatisfactory results. Since an avalanche photodiode is a purely capacitive load but this HF amplifier requires a low-impedance 50-ohm signal termination for stable operation, the amplifier must be provided with compulsory adaptation involving power consumption. At a desired amplitude of, e.g., 2 V, an HF output of 40 mW, for example, would be needed for this purpose. But this can not be achieved with the desired low power consumption.
According to the scientific paper cited above, K. Seta et al., a high-frequency transformer is to be used for signal termination matching of the necessary HF amplifier. In practice, however, this has also turned out to be problematic because only voltage ratios of 1:2 are available for the required high frequencies of the local oscillator signal (e.g., 1 GHz) and, in addition, considerable EMI problems occur due to the use of the transformer.